1. Field of the Invention
The present invention relates to the art of controlling the emissions of both particulate material and acid gases from flue gases. The term "flue gas" as used herein refers to the hot waste gas mixture which results from various acid gas producing processes such as the gas from coal-fired electric power plants and the like wherein the acid gases are sulfur oxides. The flue gas may also include an amount of small particulates commonly referred to as "fly ash".
Because of various economical and environmental factors, it has become increasingly more important for the effective and economical removal of fly ash particulates and acid gas, especially SO.sub.2 and SO.sub.3, from the flue gas. A primary benefit of such an effective and economical method of removing the constituents from flue gas is that it allows operators of coal fired plants to burn cheaper high sulfur coal while minimizing the environmental hazards of using such a fuel. This invention not only minimizes these hazards by removing the undesirable constituents from the gas, but does so with a minimum expenditure of energy which results in appreciable cost savings not achievable by other methods.
2. Description of Prior Art
Prior art methods have traditionally involved either wet scrubbing processes or dry scrubbing processes. Both of these processes are used to remove sulfur oxides from flue gas.
In the wet process, SO.sub.2 dissolves in scrubber water droplets where it reacts with a limestone slurry resulting primarily in calcium sulfite. A sulfite sludge is continuously removed from the scrubber liquid for disposal. Normally, fly ash is first removed from the flue gases by an electrostatic precipitator (ESP) prior to the scrubber to minimize the build-up of fly ash in the scrubber liquid. This process uses conventional chemical processing equipment and a low cost reagent, such as limestone; but the overall system is complex and its operation and maintenance is often difficult for electric utility operating personnel who are not experienced in chemical processes. Difficult scaling problems have been encountered and it has been difficult and expensive to dispose of the generated sludge material. These disadvantages have led to the current interest in and acceptance of the alternative dry scrubbing process which utilizes lime in a spray dryer.
In the dry scrubbing process, lime (calcium oxide) is slaked (mixed with water) to form a slurry of calcium hydroxide. The slaked lime slurry is introduced into a spray dryer through an atomizer where it contacts the hot gas. In the spray dryer, the SO.sub.2 is reacted to form calcium sulfate or sulfite while simultaneously the slurry droplets are dried. The dried particles and fly ash are next removed from the flue gas stream normally by a fabric baghouse filter. The dry scrubber method has been found to be somewhat simpler than the wet process, although the capital cost advantage generally is not too great. It does,of course, eliminate the wet sludge disposal costs and problems. However, it incurs a significant cost disadvantage by using lime instead of limestone. Lime typically costs anywhere from $50.00 to $100.00 per ton, whereas limestone only costs $5.00 to $10.00 per ton.
Both of these processes, the wet scrubber and the dry scrubber, are generally considered too expensive and too complex to operate to be suitable for use in many industrial coal-fired boiler installations.
A more recent advance in this technology is the modified dry limestone process for control of sulfur dioxide emissions illustrated in U.S. Pat. No. 3,976,747. In this process, large size particles of limestone are contacted with preconditioned flue gas in which the fly ash has been previously removed. The preconditioning of the flue gas involves both cooling and increasing the moisture content, preferably to the saturation point. This preconditioning assures that at least some condensation occurs on the limestone particles which is said to enhance the limestone reactivity with the sulfur oxides. When both the temperature and saturation temperature are preconditioned in the manner described by this patent, the scrubbing effect of the limestone remains quite high over an extended period of time and it is only necessary to periodically remove the solid product from the surface of the limestone particles. However, the preconditioning requires that the flue gas be substantially cooled by spraying water or low pressure steam into it before it passes through the limestone and, as a result, the temperature of the flue gas is reduced and this energy cannot be easily recovered.
Accordingly, there remains a need in the art for a process for treating flue gas to remove acid gas (especially sulfur oxides) and fly ash without the preconditioning steps of cooling and moisturizing so that heat energy can be conserved.